An FCC reactor, provided with a riser and a cyclone separator as above, is disclosed in U.S. Pat. No. 5,039,397. This publication describes an elongated vertical reactor vessel comprising the downstream part of a reactor riser. The reactor riser is fluidly connected to one or more primary cyclones, which are in turn connected to one or more secondary cyclones each. The cleaned gas obtained in the secondary cyclone is discharged at the upper end of the reactor vessel. The primary and secondary cyclones discharge the separated FCC catalyst particles to below into a dense fluidized bed via their respective diplegs. As fluidization gas steam is added in order to strip adsorbed and/or entrained hydrocarbons, which are present in the catalyst flow. At the lower end of this stripping zone stripped catalyst particles are discharged from the vessel and transported to a separate regenerator vessel. In the regenerator vessel coke is removed by means of combustion from the catalyst in order to provide a regenerated catalyst, which can be reused in the reactor riser.
A disadvantage of the above-described process is that the stripping efficiency of existing units is often too low.